This invention relates to crystal resonators and, more particularly, to a process for producing a plurality of high frequency crystal resonators of 30 MHz or greater.
Crystal resonators are used in a variety of timing dependent applications, such as in computers. Computers are capable of executing multiple tasks simultaneously. Yet such execution typically involves sharing buses, memory, and other common structures. Computers are therefore synchronized by a high frequency clock signal to maintain data integrity. Crystal resonators are used in computers to generate the clock signals for maintaining synchronous operations.
The crystal resonator is part of an oscillating circuit, The oscillator circuit generally comprises a piezoelectric crystal, a housing for protecting the crystal, and an amplifier-feedback loop combination capable of sustaining oscillation.
When a voltage is applied between certain faces of a piezoelectric crystal, a mechanical distortion is produced within the crystal. This phenomenon is known as the "piezoelectric effect". If the oscillator circuit provides an alternating current, the piezoelectric crystal is excited to a vibrating state at the frequency of the resonating circuit. When the oscillator circuit is energized, electrical noise will begin to excite the crystal at its natural resonant frequency. The crystal's output is then amplified and the amplified signal is fed back to the crystal. This causes the amplified signal to build up in strength at the resonating frequency of the crystal, until saturation of the circuit elements causes the overall loop gain in the circuit to fall to unity. This signal is fed to the output terminal of the oscillator.
Although a variety of piezoelectric materials may be used for resonators, monocrystalline (single crystal) quartz offers certain advantages. Single crystal quartz has low internal mechanical loss when used as a vibrator. Another important feature of quartz is that its frequency of vibration is highly stable with changes in temperature and over long periods of time.
A resonator is formed from single crystal quartz by first cutting the quartz into slabs, grinding the slabs to a desired thickness by a lapping process, and then polishing the slab surfaces. The choice of cut is usually dictated by the range of operating frequencies and the temperature range required for a particular application. Resonators with particular oblique cuts, such as AT, SC or BT, display negligible frequency variation with changes in temperature and operate at high frequencies. These resonators are generally referred to as thickness shear resonators, and are useful for making high frequency resonators on the order of 30 MHz or greater. The resonant frequency is approximately inversely proportional to the thickness of the wafer in the area of the vibration, so higher frequency devices require thinner wafers.
Single crystal quartz must be ground down to a very thin membrane to enable high resonant frequencies. However, a thin membrane is a poor structure for attaching a resonator. It is therefore desirable to produce a resonator with both a vibrating membrane region and a thicker region, the latter region serving as a support structure for attachment purposes.
Such a structure is obtained by grinding the crystal down to the thickness of the support structure, then etching the crystal to form the membrane portion.
At least two problems arise in this process. First, the crystal must be ground and polished to a highly uniform surface topography to assure successful membrane etching. Second, the precise thickness of the resonating membrane requires high precision etching. It is desirable to be able to produce the resonator from a wafer of single crystal quartz. Until now, the grinding process has not provided a quartz crystal wafer with sufficient thickness uniformity suitable for the mass-production of crystal resonators from wafers. Second, the standard etching process lacks the precision required to mass-produce resonators on a wafer with consistent frequencies. The production of crystal resonators with both an etched membrane and a support structure consequently has necessitated grinding and etching each unit individually. This process is time-consuming and costly.
One such prior art crystal resonating structure used in high frequency resonators is an "inverted mesa structure". "Inverted mesa structure" is a term of art referring to crystal resonators with a thin central membrane completely surrounded by a thicker support structure. Electrodes deposited on the membrane cause it to vibrate.
Inverted mesa structures have at least one disadvantage in addition to high production cost. The oscillating wave traveling outward from the vibrating (electrode) region of the membrane must be diminished to a very low amplitude by the time it reaches the surrounding support structure. The membrane must therefore be large relative to the electrode area to avoid undesirable damping of the resonance. Additional area is needed for the thicker supporting region, placing a physical constraint on the minimum size of the resonator.